What are the common faults and troubleshooting methods for machine tool servo systems?
The servo system is the link between the numerical control device and the machine host. It is used to receive the feed pulse or feed displacement information from the CNC device interpolator. After a certain signal conversion and voltage and power amplification, the servo motor drives the transmission mechanism. Finally, it is converted into a linear displacement or a rotary displacement of the machine table relative to the tool.
In order to improve the performance of CNC machine tools, high requirements are placed on the feed servo system for machine tools. Due to the different processing tasks performed by various CNC machine tools, the requirements for the feed servo system are also different. The following are common faults in the servo system.
1, overtravel
When the feed motion exceeds the software limit set by the software or the hardware limit set by the limit switch, an overtravel alarm will occur, and the alarm content will be displayed on the CRT. According to the CNC system manual, the alarm can be eliminated. Fault, cancel the alarm.
2, overload
When the load of the feed motion is too large, frequent positive and negative movements, and poor lubrication of the transmission chain, an overload alarm will be caused. Alarm information such as servo motor overload, overheating or overcurrent is usually displayed on the CRT. At the same time, on the feed drive unit in the power cabinet, the indicator light or the digital tube will prompt the drive unit to overload, over current and other information.
3, shake
Agitation occurs during feeding:
1 The speed measurement signal is unstable, such as the speed measuring device failure, the speed feedback signal interference, etc.;
2 The speed control signal is unstable or interfered with;
3 The terminal is not in good contact, such as loose screws.
When the turbulence occurs in the commutation moment of the forward direction and the reverse motion, it is generally caused by the backlash of the feed chain or the servo system gain being too large.
4, crawling
When it occurs in the starting acceleration section or low-speed feed, it is generally caused by factors such as poor lubrication state of the feed transmission chain, low servo system gain, and excessive load. In particular, the coupling between the servo motor and the ball screw is not synchronized with the rotation of the servo motor due to loose coupling or defects in the coupling itself, such as cracks. Give the movement a flickering and creeping phenomenon.
5, the machine tool vibrates
When the machine is running at high speed, vibration may occur and an overcurrent alarm will occur. Machine vibration problems are generally speed problems, so you should look for the speed loop; and the entire adjustment process of the machine speed is done by the speed regulator, that is, any speed-related problems, you should look for the speed regulator, so the vibration problem Look for the speed regulator. Mainly from the given signal, feedback signal and speed regulator itself to find faults.
6, the servo motor does not turn
In addition to the speed control signal, the CNC system to the feed drive unit also has an enable control signal, typically a DC24V relay coil voltage. The servo motor does not turn. Common diagnostic methods are:
1 Check whether the CNC system has a speed control signal output;
2 Check if the enable signal is on. Observe the I/O status through CRT, analyze the PLC ladder diagram (or flow chart) of the machine tool to determine whether the starting condition of the feed axis, such as lubrication and cooling, is satisfied;
3 pairs of servo motors with electromagnetic brakes should check whether the electromagnetic brake is released;
4 feed drive unit failure;
5 servo motor failure.
7, position error
When the servo axis motion exceeds the position tolerance range, the CNC system will generate an alarm with excessive position error, including following error, contour error and positioning error. The main reasons are:
1 The tolerance range set by the system is small;
2 servo system gain setting is improper;
3 position detection device is polluted;
4 The cumulative error of the feed chain is too large;
5 The balance device (such as balance hydraulic cylinder, etc.) is unstable when the spindle head moves vertically.
8, drift
When the command value is zero, the axis still moves, causing a position error. Eliminated by error compensation and zero speed adjustment of the drive unit.
9. Clearance and looseness of mechanical transmission components
In the feed chain of CNC machine tools, the transmission is often damaged due to the gap between the keyway and the key of the transmission element. Therefore, in addition to careful selection of the key coupling mechanism during design, the machining and assembly must be strictly investigated. The preload of the bearing should be checked when assembling the ball screw to prevent axial movement of the ball screw, as clearance is another cause of significant drive clearance.
